A secondary battery is an energy storage device that is basically made up of positive electrode/negative electrode/separator/electrolyte solution and can be recharged by reversible conversion of chemical energy and electrical energy, achieving high energy density, and secondary batteries are widely used in compact electronic devices including mobile phones and laptop computers. Recently, in keeping with environmental issues, high oil price, and energy efficiency and storage, application of secondary batteries to hybrid electric vehicles (HEV), plug-in EV, e-bike, and energy storage system (ESS) is expanding at a high rate.
In manufacturing and using secondary batteries, ensuring safety is an important problem to solve. In particular, a separator commonly used in electrochemical devices shows extremely severe thermal shrinkage behaviors in some situations, for example, at high temperature, due to its material properties and manufacturing process properties, causing a stability problem such as an internal short circuit. Recently, to ensure safety of secondary batteries, an organic-inorganic composite porous separator having a porous coating layer was proposed, in which the porous coating layer is formed by coating a mixture of inorganic particles and a binder resin on a porous substrate for a secondary battery separator (see Korean Patent Application No. 10-2004-0070096). However, when an electrode assembly is formed by stacking an electrode and a separator, interlayer bonding is inadequate and there is a high risk of separation between the electrode and the separator, and in this case, inorganic particles that are detached during separation may act as local defects in a device.
To solve the problem, Korean Patent Publication No. 10-2006-0116043 discloses a method in which ethanol is added to a solution in which PVDF is dissolved in a good solvent such as acetone, and the solution is coated on a separator and dried, yielding a porous adhesion layer by a phase separation effect. The porous adhesion layer obtained by the method has advantages in terms of excellent wettability and low resistance while a battery works, while due to swelling after injection in the manufacture of the battery, the porous adhesion layer has reduced bond strength with the separator, i.e., reduced mechanical strength and low cycling characteristics, and causes interlayer mixing with a porous coating layer, resulting in clogging of the pores of the porous coating layer and eventual reduction in air permeability of the separator.
Therefore, there is an urgent demand for the development of new technology to improve the adhesion of a separator and an electrode.